2 questions

[W1RKW]

Not sure if this should be posted here or not but here goes.

What is the performance effect on a transformer if one does not use all the windings in a multiple winding transformer? In other words, not using all the secondary windings but just one?

Does a variable power resistor that is rated for a certain power rating at full resistance, does the power rating change if one taps it for a lesser value of resistance? If so, how does one determine the power rating of the resistor if it is tapped less than full range?

[WQ9E]

Bob,

I will take a stab at this and hopefully you will get more information but I am afraid there are no hard and fast formulas for your transformer question.  When only using one secondary you have removed the heating effects of the additional loads and of course somewhat reduced the load on the primary so you will be able to increase the load on the single secondary in use.  The $64,000 (or $64 depending upon the value of the transformer) question is how much.  A big part of the answer to that depends upon whether the load on the secondary in use is continuous or intermittent and whether the transformer was originally rated for continuous use (like the power transformer in a television) or if it already was rated for ICAS service.  Under intermittent load (say the plate supply in for a class B SSB amplifier stage) using a TV set transformer you could probably easily draw peaks of twice the rated current without issue if neither filament winding were used.  On the other hand, if the transformer was rated for ICAS already then even removing other secondary loads equal to half of the transformer rating you still wouldn't want to go more than around 20% above rated on the secondary in use if it is powering a class C plate modulated final designed to stand up to "ole buzzard" transmissions because even with the other loads removed it is likely you will create considerable localized heating for the winding in use.

I have seen guidelines that say it is OK to load until the voltage drops X% from no load to loaded, I guess the X amount depends upon how much you want the transformer to live a long life.

The resistor question is a bit easier, I don't have it handy but I have a publication from one of the old resistor manufacturers that states you de-rate the wattage rating equal to the percentage of the resistance you have used.  In their example, a 50 watt 10K resistor used as an 8K resistor would be de-rated to 40 watts (80% of resistive winding used and thus 80% of the rating).  I think that is a very conservative rating since you will get some heatsink action from the unused part of the wire and probably at 80% of used wire 90% of rated wattage would be quite safe.  What the manufacturer doesn't want you do do is try to turn a 50 watt 50K resistor into a 50 watt 10K resistor.  It gets a little more complex if you are looking at the case of a tapped resistor such as R13 in the recently discussed Viking 2.  The "bleeder" current flows through the entire resistor and you are also drawing modulator screen current through a portion of the total resistor; however in this case you choose the wattage rating very conservatively because you don't want to burn out a bleeder resistor.  That being said, tapped wirewound resistors of high resistance are pretty darn fragile and thus are not the safest bleeder resistors.  Bill Orr advised that if you used a tapped wirewound as the main bleeder in a high voltage supply you should use a second, fairly high resistance value, untapped bleeder as a safety measure and given the number of open bleeders I have run across in transmitters I think he definitely has a point.

See what happens when you ask a Prof a simple question    Seriously, I hope this helps.

Rodger WQ9E

[W1RKW]

Hi Rodger,
From an empirical stand point you answered my questions. Thanks.

For the transformer the unused windings of the secondary would be the low voltage high current windings so I should be good there.

As for the tap type wire wound resistors I suspected as much but was not sure.  I have a 100K 100watter on the 2kV plus side that I want to tap to about 80 to 90K but do not want to go beyond it's power handling capability. I just was not sure how to derate it. I believe I should be well within the power handling range.

Thanks.

[N2DTS]

I tend to avoid resistors with taps, they always seem to fail or get crusty.
I had a bunch of old ones and they were all bad.

I would use two seperate resistors instead.

Brett

[WU2D]

Hey Frank,

How would you wire a HV power FET (with heatsink) as a variable bleeder current sink on the lowside resistor thus making a safe variable tap? I am thinking about something like a pot and a transistor and a little protection?

Mike WU2D

[WD8BIL]

On the transformer, just tape off the unused secondaries. As for current/power, the major determining factor is the wire in the secondary. It's sized for the current and temp rise ratings. Without the other secondaries working you could go a bit heavier on the one say 5-10%. The temp rise on that winding should take it.

Just be sure when you tape the other secondaries you don't short them out.

[k4kyv]

You could employ the unused LV secondaries to reinforce or buck line voltage to compensate for line voltage variations, or to boost or reduce the voltage from the other windings if they are a little too  high or too low.  Just wire the LV windings in series with the primary.  Of course, each winding has to be wired to the proper polarity to buck or boost.  This assumes they will carry at least as much current as what the primary pulls from the a.c. line, which is usually the  case with filament windings.  This was a commonly used trick back in the Novice days to get a little more voltage out of a TV power transformer when it was used as a transmitter plate transformer, if there were extra filament windings that weren't needed.

[K1JJ]

Bob's question made me think of something....

Let's say you have a standard 1KW mod transformer with one primary winding and TWO separate, unconnected secondary windings.

If we were to hook up the primary to a 1 KW set of modulator tubes with the secondaries open - then sock a Yallo, we would most likely arc out and blow the transformer windings due to super high inductive voltages - no load.

Now what if just the first the secondary winding has a load and the second one is open.  Why doesn't the second secondary winding develop huge voltages and arc out?

Is it because the primary's voltage is limited due to a the reflected load from the first secondary, thus cannot generate super high voltages back to the second unloaded secondary?


T

[KD6VXI]

Now what if just the first the secondary winding has a load and the second one is open.  Why doesn't the second secondary winding develop huge voltages and arc out?

Is it because the primary's voltage is limited due to a the reflected load from the first secondary, thus cannot generate super high voltages back to the second unloaded secondary?

T

Maybe not enough inductive "kick" with one set of secondaries loaded down.

I always figured the mod xformers blow themselves unloaded because of an inductive kick they get no load.  With ANY load on the transformer (well, any within reason), the inductive kick will be lower since it can't reach sky high potentials.

Kind of like when you change (gulp) bands from 160 to 80 on the linear, forgetting to unkey.

--Shane

[W1RKW]

Bob's question made me think of something....

Let's say you have a standard 1KW mod transformer with one primary winding and TWO separate, unconnected secondary windings.

If we were to hook up the primary to a 1 KW set of modulator tubes with the secondaries open - then sock a Yallo, we would most likely arc out and blow the transformer windings due to super high inductive voltages - no load.

Now what if just the first the secondary winding has a load and the second one is open.  Why doesn't the second secondary winding develop huge voltages and arc out?

Is it because the primary's voltage is limited due to a the reflected load from the first secondary, thus cannot generate super high voltages back to the second unloaded secondary?


T

Possibly since the mag flux in the core is being loaded because of the load on the secondary so as to dampen the overall mag field ?

[WD8BIL]

PLUS Vu, look at the turns ratio. I the mod xformer you have what?? 1:1, 2:1, 3:1.

In the power trannies filament windings it's 20:1.... step down..... how much voltage ??

[K1JJ]

PLUS Vu, look at the turns ratio. I the mod xformer you have what?? 1:1, 2:1, 3:1.

In the power trannies filament windings it's 20:1.... step down..... how much voltage ??

Roger there Buddly.


I find it interesting that we can plug in any power transformer, like a 1:1 isolation transformer  OR  a 30:1 step up  240v to 7KV power transformer and there are no secondary load precautions necessary.

However, put the same voltage on a 1:1 UNLOADED modulation  transformer primary (in the form of audio) and it goes ca-bloomy real quickly.

I must be missing something.  Maybe it has to do with 60hz power vs: audio frequencies more easily creating hysteresis core loading, thus  inductive spikes?   

T

[WQ9E]

I think the main issue is that with a properly operating (no shorted turns on primary or secondary) power transformer there is very minimal current flow through the primary (when the secondaries are not loaded).  With the modulation transformer, the modulator tubes are drawing their peak current through the primary whenever they are driven to peak input regardless of whether the secondary is loaded and thus will induce very high voltages in the unloaded secondary.  The same is true of the output transformer in a receiver if there is no load on its secondary.

You can get some interesting "effects" from measuring power transformers if you are not careful however.  Using a typical VOM to measure the resistance of a high voltage secondary will create a nice inductive "kick" from the collapsing field when the magnetizing voltage is removed which can have interesting impacts on the measuring instrument and instrument operator!

One additional note, you will often find that the resistance of one side of a center tapped secondary is a bit higher than the other.  This is due to the typical practice of winding one on top of the other so one side, although it has the same number of turns, will have slightly greater resistance due to the greater length of wire needed since the diameter of the outer winding is larger.  This could be a new source of income for those wanting to sell a line of Perfect Balance  power transformers to the audiophiles by not winding the split secondaries on top of each other.  This could be Gary's ultimate retirement plan!

Rodger WQ9E

 

[K1JJ]

I think the main issue is that with a properly operating (no shorted turns on primary or secondary) power transformer there is very minimal current flow through the primary (when the secondaries are not loaded).  With the modulation transformer, the modulator tubes are drawing their peak current through the primary whenever they are driven to peak input regardless of whether the secondary is loaded and thus will induce very high voltages in the unloaded secondary.  The same is true of the output transformer in a receiver if there is no load on its secondary.
Rodger WQ9E

That makes sense, Rodger, thanks.

So, what if one were to apply primary power a different way...to connect the 240V primary of a 7kv power transformer and rapidly key the primary on and off at a 60hz  rate with a switch. Would this simulate a similar current flow and produce these high voltage inductive kicks? 

I'm trying to understand what makes the modulator tubes unique to the situation.

T

[WQ9E]

Well,

You are looking at two slightly different scenarios.  With the modulator tubes you have a heavy current flow through the primary at an audio rate creating a large magnetic field varying at an audio rate which will induce a corresponding voltage in the secondary.

I think with your question, you are looking at treating the transformer as an inductive spark coil not unlike those used in a automobile ignition system.  With AC applied to a power transformer primary the inrush current is fairly high to create the initial magnetic flux but instead of then having the flux fall immediately it is smoothly rising and falling at the rate of the applied AC and thus does not induce a large current/voltage in the unloaded secondary.  On the other hand, if you were to connect a DC source with fairly high current capability to the primary and then rapidly keyed that you are turning your transformer (probably for a very short time!) into an inductive spark coil not unlike the breaker points and coil in a vintage auto ignition system.  The key is having the magnetic field collapse immediately instead of gradually as it does with a continuous applied AC source.

For the keying to have this effect you would have to be synchronized with the line frequency and keying at a rate synchronized with the 60 cycle per second rate so this will be a bit beyond even McElroy's CW ability   I vaguely recall reading some of the old transmitter designs where the HV was keyed (for CW) on the primary side of the plate transformer; probably not the smartest idea ever but damage to the transformer was probably not the biggest concern!  And I guess it was a bit safer than trying to connecting a key in series with the HV feed to the final 

Rodger WQ9E
 

[KD6VXI]

Looks like what I was thinking earlier was the ticket...

HOWEVER,

What happens when we use a "power tranny" as a "mod tranny"?  Would the same thing happen (anyone have a 'spare' they want to donate to the expiriment? )?

If not, I wonder if they use a different style of winding or iron inside the different transformers, different gaps, etc (nope, never pulled a mod transformer apart, aside from a 10 watter from an old tubed rig, and it was SO small, nothing inside could really be gleaned... Could be that I was YOUNG when I did it as welll).

--Shane

[K1JJ]

OK Rodger -

Yes, the DC would draw more current than the AC cuz of the increased inductive reactance generated as freq goes higher.

So a good way to destroy any transformer would be to rapidly key the primary w/o a secondary load -  using DC... :-0



Shane:  As far as using power transformers as modulation iron... I did that back in the 70's a few times. I couldn't find a conventional mod transformer.  I had homebrew quad 4-400A'a modulating quad 4-400A's using a T-368 power transformer in p-p Heising config. I believe it worked out to be a 4:1 transformation. It sounded just "OK."   Anyway, after a few hours it blew. I had a second one and figured the first was an anomoly. The second blew out too. What a waste. I could use those transformers in my plate supply to this day.

Yes, they can be popped just like mod iron I'm afraid.

T

[WQ9E]

Using a power tranny as a mod tranny would result in the identical pyrotechnics with an unloaded secondary.  I believe there are a number of articles on using/rewinding power transformers as mod transformers; doesn't HLR have an article on that somewhere.  Apparently the 400hz military power transformers were briefly very popular as modulation transformers since they were very cheap in surplus.  The transformer principle remains the same and the difference is in core materials and windings to achieve the desired frequency response.  

As a "small" example, one of my CE-20A transmitters is running with filament transformers in place of its original little modulation transformers (yes, even little phasing transmitters have equally little modulation transformers).  One of the two transformers had an open winding when I got it and I replaced both with a pair of matching filament transformers from Mouser.  

Rodger WQ9E

[W7XXX]

In my experience concerning audio transformer applications such as modulation and output, a transformer with 2 secondary winding will function fine as long as one of the secondary winding has a proper load. Example: Leaving the screen winding open on an ART 13 xfmr and just using the plate secondary winding has never presented a problem for me. Mod xfmrs with 500 ohm separate winding work fine with no load on the 500 ohm winding as long as the secondary plate winding has a load. Same with audio transformer both interstage and output, as long as one set of secondary winding is used, no problems. Same with multiple coil primaries in my experience.

I see no reason a 100k 100 watt resistor tapped at 80k still isn't good for 100 watts or so close it doesn't matter. It still has most of the length of the resistor for dissipation. Tapping it at 10k might reduce the wattage because of loss of length and the dissipation factor. I have bought fixed tapped 100 watt power resistors in the past that were tapped at 1/4, 1/2, 3/4 specified resistance and the spec say all taps are good for 100 watts. Prehaps they have heavier gauge wire on the lower taps. They are sealed so I don't know and often wondered how this was possible because of physical length and dissipation factor, but never could get the courage to break one open and find out. One important point that some may not know and that is if using a variable tap on a wirewound power resistor as a bleeder, always hook the tap to the unused end of the resistor and don't tighten the clamp so tight as to damage the windings. I agree that it is best to use a fixed tap or best yet 2 resistors.

[WQ9E]

W7XXX is correct in that as long as you are using nearly the entire resistor and you have chosen the power rating conservatively it will be fine.  But the manufacturers definitely de-rate based upon the amount of the total resistance used.

These are excerpts from Ohmite's spec sheet for their 210 series tapped wire wound power resistors:

Specifications:

Adjustability is 10% to 90% of full
value. Wattage is proportional to
this adjusted resistance value 

AND:

Power rating: Based on 25°C
free air rating. The stated wattage
rating applies only when the
entire resistance is in the circuit.
Setting the lug at an intermediate
point reduces the wattage rating
by approximately the same
proportion. Example: If the lug is
set at half resistance, the wattage
is reduced by approximately
one-half.


The full sheet is available at:  http://www.ohmite.com/catalog/pdf/210_series.pdf

On edit:  The case of fixed tap resistors is different because the manufacturer knows tap placement in advance and can either design it for full ratings from any tap or specify exact power ratings per section; with the adjustable units the manufacturer cannot anticipate the exact application.